1. Field of the Invention
The present invention relates generally to opto-mechanical beam control systems and more specifically to a laser beam control system that may for example be advantageously used with a laser employed as an adjunct to a heater for the melting of silicon in a controlled fashion for semiconductor crystal growth.
2. Prior Art
The use of lasers as a source of heat in growing crystals from a melt for the fabrication of semiconductor crystals is described generally in a paper by the present inventor. This prior art is entitled "GROWTH OF SINGLE CRYSTALS OF SEMICONDUCTORS BY ZONE MELTING WITHOUT A CRUCIBLE WITH LASER HEATING", published in the Soviet Physics Crystalography Journal, March-April, 1991, pages 281 and 282. This paper discusses the use of laser heating in crystal growth processes where there is a necessity for additional local heating in addition to a main source, such as an induction heating device. There is disclosed the use of an optical mechanical device fixed to the emitter of the laser and enabling one to focus the laser radiation to move the focal spot over a heated object and to effect continuous monitoring of the aiming of the invisible infrared beam at a given point on the heated object. The focal spot of the laser beam may be altered by moving a focusing objective lens in two mutually perpendicular directions in a plane perpendicular to the beam. Self-generated or reflected radiation from the heated object is directed into a dioptric tube with the aid of a plane parallel spectral divider mirror, transparent at the laser wavelength of 1.06 micrometers and fixed obliquely between the laser and the focusing objective lens. This permits an image of the heated object to be formed in the focal plane of the eyepiece of the dioptric tube, allowing an eyepiece cross-wires to be used to indicate the point of incidence of the laser beam on the object.
In an earlier paper entitled "APPLICATION OF LASER AND INDUCTION HEATING FOR SEMICONDUCTOR CRYSTAL GROWTH", by the inventor and a co-author V.I. Dobrovolskaya published in the Russian language in Elektrotekhnika, Volume 58, No. 12, pages 16-19, 1987, there is a more detailed description of the same optomechanical beam control system for accomplishing the aforementioned laser beam control for heating. As this paper describes, such a beam control system should focus the laser radiation and permit one to change the focal point position along the beam centerline. This feature is particularly important when growing crystals of various diameters. Such a beam control system should also allow one to direct the laser beam during the growth process into a selected point of the object to overcome the prior art limitation where this function has been traditionally solved by displacing the object, such as by means of a coordinate position table. Such a solution in the prior art would be difficult or impossible to carry out with a growing crystal.
As shown in FIG. 1 of the 1987 article by Dobrovolskaya and Mandel, the beam control system disclosed therein comprises a lens which is mounted for displacement along the beam centerline to provide shifting of the focal point. Furthermore, the lens is displaceable in two perpendicular directions in a plane perpendicular to the laser beam which enables deflecting the beam to the selected point of the object. This same lens also receives the self-generated or reflected radiation from the object which is passed to a plane parallel plate, a turnable mirror, a light filter and a telescope. This prior art optical train unfortunately has a number of disadvantages. One such disadvantage is that the lens used to control the position of the beam where it is incident on the crystal being grown, does not provide satisfactory accuracy in beam control, particularly when one considers that this lens is also relied on to vary the focal point of the laser beam by translation along the direction of the beam. Furthermore, movement of the lens either along the beam of the laser or perpendicular thereto to control the focal point and position of the point of incidence on the object, may detrimentally affect the imaging accuracy of the radiation of the heated object and thus decrease the accuracy with which one can observe the effect of the laser during the operation. This disadvantage is particularly significant with respect to translation of the lens for laser beam focal point control since such translation may affect the imaging capability of the lens with respect to the reflected light in which there is no compensation in the lens or any other portion of the light monitoring system of the beam control unit. Also significant is the fact that by limiting the control of the focal point of the laser by only a single lens, one loses a degree of control flexibility that would be available with a more complex lens configuration. However, by increasing the complexity of the lens configuration of the focal point control using the optical train of the prior art, one would incur the increased difficulty of maintaining an accurate and precise image of the light emission and/or reflection of the heated object. Furthermore, there is an inherent difficulty in using remote control of laser beam parameters and monitoring parameters concurrently.
Accordingly, there is a continuing need for an improved beam control system which permits more flexible control of laser beam parameters, including shape, sharpness and focal point, without detrimentally affecting the imaging portion of the system so that precise positioning and shape, sharpness and focal point control of the beam can be obtained without losing the efficacy of the light imaging needed to monitor and control the position of the laser beam precisely.